Beverage preparation machines have been known for a number of years. For example, U.S. Pat. No. 5,943,472 discloses a water circulation system between a water reservoir and a hot water or vapour distribution chamber of an espresso machine. The circulation system includes a valve, metallic heating tube and pump that are connected together and to the reservoir via different silicone hoses, which are joined using clamping collars.
EP 1 646 305 discloses a beverage preparation machine with a heating device that heats circulating water which is then supplied to the inlet of a brewing unit. The brewing unit is arranged to pass heated water to a capsule containing a beverage ingredient for its brewing. The brewing unit has a chamber delimited by a first part and a second part movable relative to the first part and a guide for positioning a capsule in an intermediate position between the first and second parts before moving the first and second parts together from an open to a closed configuration of the brewing unit.
In-line heaters for heating circulating liquid, in particular water are also well known and are for example disclosed in CH 593 044, DE 103 22 034, DE 197 11 291, DE 197 32 414, DE 197 37 694, EP 0 485 211, EP 1 380 243, EP 1 634 520, FR 2 799 630, U.S. Pat. Nos. 4,242,568, 4,595,131, 4,700,052, 5,019,690, 5,392,694, 5,943,472, 6,246,831, 6,393,967, 6,889,598, 7,286,752, WO 01/54551 and WO 2004/006742.
One problem with heaters for heating water in beverage machines lies in the operating temperature, i.e. usually close to the boiling temperature of water, that favours scale deposition from the water in the heating device. Especially in-line heaters having a heated duct or channel for quickly heating up circulating water are exposed to such scale depositions that may end up clogging the in-line heater.
This problem is of particular relevance with thermoblocks that are widely used in beverage preparation machines.
A solution to this problem has been provided with descaling procedures during which a descaling agent is circulated in the machine's fluid circuit. However, such a procedure may last a significant period of time, e.g. 0.5 to a few hours, and requires the attention of the user or of a service person.
Thermoblocks are in-line heaters through which a liquid is circulated for heating. They comprise a heating chamber, such as one or more ducts, in particular made of steel, extending through a (massive) mass of metal, in particular made of aluminium, iron and/or another metal or an alloy, that has a high thermal capacity for accumulating heat energy and a high thermal conductivity for the transfer the required amount of the accumulated heat to liquid circulating therethrough whenever needed. Instead of a distinct duct, the thermoblock's duct may by a through passage that is machined or otherwise formed in the duct's body, e.g. formed during a casting step of the thermoblock's mass. When the thermoblock's mass is made of aluminium, it is preferred, for health considerations, to provide a separate duct, for example of steel, to avoid contact between circulating liquid and aluminium. The block's mass can be made of one or several assembled parts around the duct. Thermoblocks usually include one or more resistive heating elements, for instance discrete or integrated resistors, that convert electrical energy into heating energy. Such resistive heating elements are typically in or on the thermoblock's mass at a distance of more than 1 mm, in particular 2 to 50 mm or 5 to 30 mm, from the duct. The heat is supplied to the thermoblock's mass and via the mass to the circulating liquid. The heating elements may be cast or housed into the metal mass or fixed against the surface of the metal mass. The duct(s) may have a helicoidal or another arrangement along the thermoblock to maximise its/their length and heat transfer through the block.
To be operative to heat-up circulating water from room temperature to close to the boiling temperature, e.g. to 98° C., a thermoblock needs to be preheated, typically for 1.5 to 2 minutes. To reduce the waiting time between two consecutive beverage preparation cycles, such a thermoblock is maintained at its operative temperature. However, such a process consumes a significant amount of energy to be ready at any time, especially when successive beverages are requested with significant time gaps between them. With the trend to more environmentally friendly appliances and energy saving, beverage preparation machines include timers for shutting of the machines or entering a standby mode automatically, for instance as discussed in WO 2009/092745 and in EP 09168147.8.
Instant heating heaters have been developed and marginally commercialised in beverage preparation machines. Such heaters have a very low thermal inertia and a high power resistive heater, such as thick film heaters. Examples of such systems can be found in EP 0 485 211, DE 197 32 414, DE 103 22 034, DE 197 37 694, WO 01/54551, WO 2004/006742, U.S. Pat. No. 7,286,752 and WO 2007/039683.
These instant heaters are however expensive and require a sophisticated and highly accurate control system to avoid local hot spots. An accurate control of the power of such heaters is difficult to achieve without coming into conflict with flicker standards.
Thus, there is still a need to provide a simple, ecological, inexpensive and reliable heating system for a machine for preparing hot beverages, such as tea or coffee.